1. Field of the Invention
The present invention relates to a method for bleaching textile pieces with reducing agents, in particular denim pieces or warp bodies comprised of indigo-dyed warp and white shot.
2. Description of the Related Art
Inspired by modern processing technology and driven by the will for stylish textile designs by means of washing, bleaching and dyeing, there has never been a lack of attempts to vary denim pieces. Denim is a weaving technique wherein the warp consists of a yarn dyed with a continuous-indigo-dyed yarn and the shot of a white yarn. Blue denim, a fabric often used for producing blue jeans is a three-leaf warp body (K2s/1) for example, which has a warp which is dyed blue by means of indigo mainly on the fabric surface. The mainly white-dyed shot is visible on the underside of the fabric in contrast thereto.
For this reason, industrial laundries have continuously attempted to perform, by means of outlandish technologies, a discharge in a mechanical (stone-wash) or chemical (snow-wash) manner. A typical method used by industrial laundries for producing stylish jeans pieces by means of the stone-wash method proceeds as follows:
The finished ready-to-wear pieces are turned inside out and pre-washed or desized. The pieces are then removed from the washing machine, turned right side out and are washed with calcareous sandstone (pumice stone) at a ratio of 1:3 (1 kg material:3 kg stone). The material is then removed from the machine, the stones are removed and it is bleached with sodium hypochlorite (depending on the desired shade) (Peter, M.; Ruette, H. K., Grundlagen der Textilveredelung [Basics of Textile Finishing], 13th ed., Deutscher Fachverlag, 1989, pp. 80 to 81).
In the jargon of textile finishing, this type of processing of ready-to-wear goods comes under the term "fully fashioned" finishing.
In accordance with DE-A1-38 33 194, when dyeing textile materials made of cellulose fibers or containing cellulose fibers mixed with synthetic fibers, vat dyestuffs in aqueous alkaline medium in the presence of reducing agents and, if required, further conventional auxiliary agents, are used at temperatures of, for example, 45.degree. to 60.degree. C. Dyeing is subsequently completed in that the textile material is rinsed, oxidized and washed.
The structural principle of indigoid dyestuffs can be generally described by the following formula: ##STR1## D.sub.1 =D.sub.2 =NH=indigoid dyestuff in the narrower sense D.sub.1 =D.sub.2 =S=thioindigoid dyestuff
R.sub.1 to R.sub.4 =structural elements PA1 A. Sodium dithionite and/or formamidine sulfinic acid and PA1 B. alpha-hydroxycarbonyl compounds at a weight ratio of 1:1 to 1:15 are used as reducing agent mixtures and dyeing is performed at pH values of at least 13 and at temperatures above 75.degree. C.
The installation of these structural elements. R, D and CO in heterocyclic rings results in stable, technically usable dyestuffs, such as the classical indigo, which is registered in the Color Index under the identification C.I. vat blue 1 or C.I. pigment blue 66, and is commercially available.
The numerous representatives of indigoid dyestuffs are being traded as so-called vat dyestuffs, less frequently as pigments. The dyestuffs obtainable by the direct halogenation of indigo have particularly proven themselves, such a C.I. vat blue 41, C.I. vat blue 5, C.I. vat blue 37, C.I. vat blue 35, C.I. vat blue 48 or C.I. acid blue 74. Blue tones with a greenish cast are created if, for example, indigo is chlorinated or if hypochlorite is used for bleaching.
Vat dyestuffs are practically insoluble in water and must be made water-soluble prior to dyeing by reduction in an alkaline solution. The reaction product obtained, also called leuco base or vat salt, is absorbed by the substrate and now can be reoxidized to the dyestuff. Vat dyestuffs therefore contain structural elements which, in the oxidized form, make the molecule insoluble in water and, in the reduced form, which can be obtained reversibly, make it soluble in water.
Up to now, the removal of these vat dyestuffs from denim pieces by the employment of hypochlorite or bleaching lye was customary. Based on the order by the Federal Ministry for Agriculture and Forestry regarding the Limitation of Waste Water Emissions from Textile Finishing and Processing Plants, Federal Law Gazette No. 612 of Sep. 24, 1992, it is required, for example in accordance with Sect. 33a of the Water Rights Law (WRG), to avoid hypochlorite. The free chlorine content of the waste water of these plants, calculated as Cl.sub.2, is not permitted to exceed 0.2 mg/l, and absorbable, organically bound halogens (AOX), calculated as Cl, are not permitted to exceed 0.5 mg/l. In the course of the conventional production of stone-washed denim, considerable amounts of active chlorine remain in the bleaching bath. Organic molecules are created during chlorination of the vat dyes, which are removed along with the washing bath and are identified as AOX in the waste water.
The use of hypochlorite liquor furthermore has the disadvantage that some vat dyestuffs result in a blue with a greenish cast when the dyestuff molecule is chlorinated.
Customary methods used in textile dyeing cannot be employed for bleaching denim pieces. With dyeing, the aim is to obtain the most even coloration possible and an equal absorption of the dyestuff by the fibers. Up to now it was possible to make the vat dyestuffs water-soluble in the form of a leuco base by means of reducing agents and also mixtures of reducing agents. However, the dyestuff is simultaneous absorbed by the fibers. Sodium dithionite, hydroxymethane sulfinic acid, thiourea dioxide (formamidine sulfinic acid) or mixtures of these compounds are described as the usual reduction agents. Too strong reduction agents cannot be used, because it is possible that the dyestuff becomes over-reduced and destroyed (DE-A1-20 11 387). In accordance with DE-A1-38 22 194, alpha-hydroxycarbonyl compounds are also being considered in order to avoid over-reduction.
However, for bleaching denim pieces, the aim is to remove the dyestuff from the indigo-dyed warp and to prevent its absorption by the white shot (coloring). The shade is supposed to become lighter, but is not to be changed in depth. It is necessary to prevent AOX and active chlorine in the waste water.
It is therefore the object of the invention to avoid the above mentioned disadvantages, to produce evenly lightened denim pieces without coloring the white shot and changing the depth of shade. Chlorine and organic chlorine compounds in the waste water are to be prevented.